Exploratory marine drilling operations are generally conducted from fixed, anchored or dynamically positioned floating offshore platforms--such as jack-up rigs, semi-submersibles and drill ships. Drilling operations conducted from these vessels, such as a drill ship, are normally conducted through a moon pool or a drillway through which drilling and other assemblies are passed. The dimensions of the moon pool, especially in the case of drill ships wherein the moon pool is usually located in the center of the ship's hull, necessarily limits the size of equipment that can be passed therethrough as equipment is lowered from and raised into the vessel.
When drilling at a subsea site where unstable soil conditions exist, for example, where the underwater bottom is comprised of unconsolidated silt or clay, the size of the moon pool presents a problem. Namely, a drilling sub-base dimensioned to pass through the moon pool may not provide sufficient bearing area to support drilling operations into the soft underwater bottom. To explain more fully, in normal offshore drilling operations, after the drill ship has been positioned above the drilling site, a drilling sub-base, having an aperture extending therethrough for passage of a drill bit, is lowered from the vessel and set into position on the underwater bottom. The drilling sub-base provides the necessary guidance means for drilling of a surface hole into the sea floor. After the surface hole has been drilled to an adequate depth, which is dependent upon the underwater bottom soil conditions, a foundation pile having a permanent drilling base fixedly connected thereto is lowered into the surface hole through the aperture in the drilling sub-base. The permanent drilling base is seated on the drilling sub-base and has its weight supported by the sub-base. The foundation pile is cemented in the surface hole to provide the foundational support necessary for further drilling. When drilling at an underwater site having a soft bottom that provides a weak foundational footing, drilling conducted through a drilling sub-base sized to pass through the drill ship's moon pool will likely cause a collapse or "wash-out" of the soil beneath the drilling base. This is because drilling fluid and cuttings produced by drilling of the surface hole flow to the top of the hole to erode the soil beneath the sub-base or to case the sides of the hole to slough away. When this occurs, the opening at the top of the surface hole becomes larger than the sub-base, resulting in the drilling sub-base literally falling into the surface hole. This will prevent further drilling.
In the past where soil conditions have required the use of a drilling sub-base of larger cross-sectional bearing area than that which can be lowered through a drilling vessel's moon pool, it has been the practice to "keel-haul" or tow the larger sub-base to the drilling site. This is an unsatisfactory solution to the problem as it entails an expensive and time-consuming operation. Accordingly, the present invention is directed to a drilling sub-base assembly which is capable of easily passing through a moon pool of a drilling vessel and which is capable of providing the needed foundational support for conducting drilling operations into a soft underwater bottom.